1. Field
Disclosed herein is a tire having a radial carcass reinforcement and more particularly to a tire intended to equip vehicles carrying heavy loads and running at sustained speed, such as, for example, lorries, tractors, trailers or buses.
2. Description of Related Art
Generally, in the tires of heavy-duty type, the carcass reinforcement is anchored on either side in the region of the bead and is surmounted radially by a crown reinforcement composed of at least two superimposed layers formed of threads or cords which are parallel in each layer and crossed from one layer to the next, forming angles of between 10° and 45° with the circumferential direction. The said working layers, forming the working reinforcement, can also be covered with at least one “protective” layer formed of reinforcing elements which are advantageously metallic and extensible, referred to as elastic. It can also comprise a layer of metal threads or cords having a low extensibility forming, with the circumferential direction, an angle of between 45° and 90°, this “triangulation” ply being radially located between the carcass reinforcement and the first “working” crown ply, which are formed of parallel threads or cords exhibiting angles at most equal to 45° in absolute value. The triangulation ply forms, with at least the said working ply, a triangulated reinforcement which exhibits, under the various stresses to which it is subjected, few deformations, the triangulation ply having the essential role of absorbing the transverse compressive loads to which all the reinforcing elements in the region of the crown of the tire are subjected.
Cords are said to be inextensible when the said cords exhibit, under a tensile force equal to 10% of the breaking force, a relative elongation at most equal to 0.2%.
Cords are said to be elastic when the said cords exhibit, under a tensile force equal to the breaking load, a relative elongation at least equal to 3% with a maximum tangent modulus of less than 150 GPa.
Circumferential reinforcing elements are reinforcing elements which form, with the circumferential direction, angles within the range +2.5°, −2.5° in the vicinity of 0°.
The circumferential direction of the tire, or longitudinal direction, is the direction corresponding to the periphery of the tire and defined by the direction in which the tire runs.
The transverse or axial direction of the tire is parallel to the axis of rotation of the tire.
The radial direction is a direction which intersects the axis of rotation of the tire and is perpendicular thereto.
The axis of rotation of the tire is the axis around which it revolves in normal use.
A radial or meridian plane is a plane which contains the axis of rotation of the tire.
The circumferential median plane, or equatorial plane, is a plane perpendicular to the axis of rotation of the tire and which divides the tire into two halves.
The term “modulus of elasticity” of a rubber mixture is understood to mean a secant modulus of extension at 10% deformation and at ambient temperature.
As regards the rubber compositions, the measurements of modulus are carried out in tension according to Standard AFNOR-NFT-46002 of September 1988: the nominal secant modulus (or apparent stress, in MPa) at 10% elongation is measured in second elongation (i.e., after an accommodation cycle) (normal conditions of temperature and hygrometry according to Standard AFNOR-NFT-40101 of December 1979).
Some current tires, referred to as “road” tires, are intended to run at high speed and over increasingly long journeys, as a result of the improvement in the road network and of the growth of the motorway network throughout the world. The combined conditions under which such a tire is called upon to run without any doubt makes possible an increase in the number of miles travelled, the wear on the tire being reduced; on the other hand, the endurance of the tire and in particular of the crown reinforcement is detrimentally affected.
This is because there exist stresses at the crown reinforcement and more particularly shear stresses between the crown layers, combined with a not insignificant rise in the operating temperature at the ends of the axially shortest crown layer, the consequence of which is the appearance and the propagation of cracks in the rubber at the said ends.
In order to improve the endurance of the crown reinforcement of the type of tire studied, solutions relating to the structure and quality of the layers and/or profiled elements of rubber mixtures which are positioned between and/or around the ends of plies and more particularly the ends of the axially shortest ply have already been introduced.
It is known in particular to introduce a layer of rubber mixture between the ends of the working layers in order to create a decoupling between the said ends in order to limit the shear stresses. Such decoupling layers must, however, exhibit a very good cohesion. Such layers of rubber mixtures are, for example, described in Patent Application WO 2004/076204.
Patent FR 1 389 428, in order to improve the resistance to deterioration of the rubber mixtures located in the vicinity of the crown reinforcement edges, recommends the use, in combination with a tread of low hysteresis, of a rubber profiled element covering at least the sides and the marginal edges of the crown reinforcement and consisting of a low-hysteresis rubber mixture.
Patent FR 2 222 232, in order to prevent separations between crown reinforcement plies, teaches coating the ends of the reinforcement in a rubber mat, the Shore A hardness of which is different from that of the tread surmounting the said reinforcement and greater than the Shore A hardness of the rubber mixture profiled element positioned between the edges of crown reinforcement plies and carcass reinforcement.
The tires thus produced make it possible effectively to improve the performance, in particular in terms of endurance.
Furthermore, it is known, in order to produce tires having a very broad tread or else in order to confer, on tires of a given dimension, higher load capabilities, to introduce a layer of circumferential reinforcing elements. Patent Application WO 99/24269 describes, for example, the presence of such a layer of circumferential reinforcing elements.
The layer of circumferential reinforcing elements is normally composed of at least one metal cord wound in order to form a turn, the angle at which it is laid with respect to the circumferential direction being less than 2.5°.